The present invention relates to an apparatus for measuring the flow of intake air to an engine. The apparatus is provided with a heater-type, air-flow sensing device containing a heat resistive element of which the radiation coefficient is dependent on an air flow in an intake pipe, and of which heating is controlled by current flowing therethrough. Further, the heat resistive element is designed for sensing the flow rate of intake air to an engine as one of the running conditions of the engine, as used for an electronic type engine control unit for controlling an injected amount of fuel through an arithmetic operation of a fuel injection amount.
An engine control unit using a microcomputer as a means for electronically controlling the engine has been known. In this type of the control unit, running conditions of the engine are always monitored, and the monitored data are fed as measured data to the engine control unit.
The means for sensing engine running conditions contains, for example, a sensor for sensing engine speed, a sensor for sensing the temperature of cooling water in the engine, a sensor for sensing the closing of a throttle, and a sensor for measuring an amount of intake air, which is directly related to the engine running conditions. The sensed signals by these sensors are transferred to the microcomputer. Then, the microcomputer calculates a fuel injection amount, an ignition timing, etc., on the basis of the sensed signal.
Many types of intake air flow sensors in use for collecting sensed data for the engine control have been known. Of these sensors, a heater-type, air-flow sensor, which senses a flow rate of intake air in terms of an electrical signal, is known.
An air flow measuring apparatus of the heater type is known and disclosed in Japanese Patent Publication No. 55-57112 (West Germany Patent Application Nos. P2845661.3 and P2911631.2). This apparatus contains a resistive element of which resistance depends on temperature in an intake pipe. Current is fed to the resistive element for controlling its heating. In this case, the resistive element is placed in the flow of air, and its radiation effect is selected corresponding to the air flow. Resistance of the resistive element of which the heating is thus controlled is selected according to its temperature. Temperature of the resistive element is monitored by monitoring its resistance. On the basis of the result of the monitoring, the heating current fed to the resistive element is controlled so that the temperature of the resistive element is kept constant.
Thus, in such an air flow measuring apparatus, the heating current value necessary for keeping the resistive element at a fixed temperature corresponds to an amount of air flow in the intake pipe. Accordingly, the air flow signal represents the heating current.
The measured output signal, derived from the heating-type, air-flow measuring apparatus, takes a signal form of nonlinear analog voltage.
In the engine control unit, it is required to control an injected amount of fuel in a broad range from 50-100 times, with a strict tolerance of several %. A measuring signal of the flow rate of intake air to the engine control unit executing such a control must be very high. Further, since the engine control unit is based on a microcomputer, the air flow measuring signal in the analog form is converted into a digital signal before it is supplied to the microcomputer. The high accuracy of the measuring signal requires a high accuracy A/D converter, as disclosed in Japanese Patent Publication No. 56-24521.